In communication systems, data (or signal) is transferred over a physical media such as, for example, twisted pair wire, coaxial cable, or optical fiber. These physical media are non-ideal communication channel, which degrades the signal by causing attenuation and delay. The physical media further degrades the signal by adding noise and distortion to the signal.
FIGS. 1 and 2 illustrate a typical transmitter and a receiver, respectively, of a wireline communication system. The blocks in the transmitter 100 and receiver 200 are connected differentially as indicated by the double lines. Referring now to FIG. 1, a transmitter 100 includes a jitter attenuator (“JA”) 104, a digital encoder (“encoder”) 108, a phase locked loop (“PLL”) 112, a trapezoidal pulse generator (“TPG”) 116, a line driver (“LD”) 120, and a cable 124. In the following description, the physical media will bc referred to as the cable.
In operation, a clock signal and data is provided to the JA 104. The JA 104 removes unwanted jitter from the clock signal, and synchronizes the data and the clock signal. The JA 104 provides the data to the encoder 108. The encoder 108 encodes the data according to a standard coding scheme.
The JA 104 provides the clock signal to the PLL 112. The PLL 112 multiplies the clock signal to generate a higher frequency clock signal in order to meet the over-sampling requirement of the TPG. In this case, the PLL 112 multiplies the clock signal by 4, i.e., multiplies the clock frequency by 4. The higher frequency clock signal is received by the TPG. The TPG 116 also receives the encoded data from the encoder 108.
The TPG 116 converts the digital signal into an analog signal that is suitable for transmission. The output of the trapezoidal pulse generator 116 is received by a line driver 120. The line driv& 120 drives the resistive load of a media 124 such as a coaxial cable. In other words, the line driver transmits the analog signal over the coaxial cable 124.
Referring now to FIG. 2, a receiver 200 includes a variable gain aniplifer (VGA) 204, an equalizer 208, a peak detector (PD) 216, a slicer 240, an analog offset controller (AOC) 212, a clock and data recovery circuit (CDR) 220, an automatic equalizer controller (AEC) 224, an analog gain controller (AGC) 228, and a digital decoder 232.
The signal transmitted over a cable 202 is received by the receiver 200. The VGA 204 amplifies the signal to compensate for the frequency-independent loss, also known as resistive loss or flat loss.
The output of the VGA 204 is received by the equalizer 208. The equalizer 208 compensates for the frequency-dependent loss on the cable also known as cable loss. The equalizer 208 boosts the high frequency components of the signal to compensate for the cable loss.
The output of the equalizer 208 is received by the PD 216. In general, the PD 216, which receives an analog output from the equalizer 208, determines the peak of the equalized signal. The output of the equalizer 208 is also received by the AOC 212, which controls through the VGA 204 the differential offset of the receiver. Thus, the AOC 212 forms a feedback loop to adjust through the VGA 204 the differential offset of the receiver, driving the differential offset to 0V level. The differential offset of the receiver is driven to a 0V level in order to eliminate harmonic distortion inside the receiver 200.
As discussed before, the output of the equalizer 208 is received by the PD 216. The peak detector determines the peak of the equalized signal (i.e., the output of the equalizer 208) and sends the peak value to the slicer 240. The slicer 240 also receives the output of the equalizer 208. The slicer 240 functions as an analog to digital converter (e.g., a 2 bit A/D converter), which outputs a digital signal using the peak value.
The digital output of the slicer 240 is received by the CDR 220. The CDR 220 extracts the correct clock signal and data from the digital signal and also synchronizes the data and the clock signal. The output of the CDR 220 is received by the decoder 232, which decodes the signal according to a standard decoding scheme.
The analog output of the PD 216 is received by the AGC 228, which controls the gain of the VGA 204. The digital output of the slicer 240 and the output of the CDR 220 are received by the AEC 224, which controls the gain of the equalizer 208 by adjusting the equalizer coefficients or steps.
A signal, such as, for example, a digital signal (i.e., data) can be decomposed into a set of sinusoidal waves according to the Fourier theorem, each sinusoidal wave having a different frequency and amplitude. As discussed before, when a signal such as a sinusoidal wave travels over a cable, which is a non-ideal communication channel, it suffers from attenuation. The attenuation refers to the loss or decrease in amplitude of the signal. The sinusoidal wave is also subjected to a delay when traveling through the cable. The delay varies depending on the frequencies. Thus, the sinusoidal waves that form a digital signal will each be delayed by a varying degree. In general, the sinusoidal wave's attenuation and delay is directly proportional to the transmission distance. Thus, the longer the sinusoidal wave travels, the more attenuation and delay it suffers.
FIG. 3 illustrates the attenuation and delay of a square wave that travels through a 100 feet cable and a 1000 feet cable, respectively. As shown in FIG. 3, the square wave's attenuation and delay increase as the wave travels longer distance.
A signal traveling through a cable is subjected to two types of loss: (a) cable loss; and (b) flat loss. A flat loss causes all frequencies of the signal to be attenuated by the same level. A cable loss causes different levels of attenuation to different frequencies. In general, a cable causes higher level of attenuation to high frequency signals and lower levels of attenuation to low frequency signals. Thus, when a square wave travels through a cable, its higher frequencies are attenuated more than lower frequencies. The cable also adds noise and distortion to the signal, which causes degradation of the signal.
In general, the receiver 200 (also known as an analog front end receiver) is relied upon to restore transmitted signals after they travel through the cable. As shown in FIG. 2, the receiver 200 provides analog gain compensation, equalization and filtering. The equalizer 208 compensates for the frequency-dependent loss on the cable. The equalizer 208 is a combination of several blocks, each block having a low pass filter and an adjustable zero. The low pass filter is implemented with a fixed pole. In a typical equalizer, an all 0 code corresponds to a scenario when the pole and zero of each block are aligned and their individual effects are cancelled. When the equalizer coefficient is increased, the zero location moves towards lower frequency, resulting in a high pass function. The purpose of the equalizer is to create a high-pass function that is inverse of the low pass function of the cable, so that the combined frequency response of the cable and the equalizer is 0 dB (i.e., flat).
Ideal Equalization
FIGS. 4(a) and 4(b) illustrate an ideal equalization scheme, which restores a signal to its original state. In FIG. 4(a), the signal originates from a transmitter 404, travels through a cable 408, and is received by a receiver 412. The signal is subjected to cable loss as it travels through the cable 408. FIG. 4(a) shows that the original waveform is attenuated due to cable loss and is then restored or equalized by an equalizer inside the receiver 412.
FIG. 4(b) illustrates the cable loss characteristics, the equalization frequency response, and the overall transfer function. The cable loss characteristics indicate attenuation of the high frequency components of the signal due to the low pass characteristics. The equalizer is adjusted so that the high frequency components of the signal are amplified, thereby canceling the cable loss effect and restoring the signal to its original state. The overall transfer function after the equalization is the 0 dB line.
In reality, however, the equalizer doesn't completely cancel the effect of the cable loss on a signal. The equalizer either under equalizes or over equalizes.
Under Equalization
If the equalizer's high frequency boost is inadequate to cancel the loss caused by the cable, the resulting signal will be under equalized. An under equalized signal is not fully restored to its original form. FIGS. 5(a) and 5(b) illustrate an under equalization scenario. In FIG. 5(a), a signal originating from a transmitter 504 travels over a cable 508 and is subjected to cable loss. The cable loss causes attenuation of the high frequency contents of the signal. An equalizer inside the receiver 512 attempts to restore the signal to its original shape. However, as shown in FIG. 5(a), the equalizer's high frequency boost is inadequate to restore the attenuation due to the cable loss. Thus the signal after equalization is under equalized. FIG. 5(b) shows the equalization frequency response, cable loss characteristics and overall transfer function. Due to inadequate equalization, the transfer function indicates that the high frequency components are not completely restored.
Over-Equalization
If the equalizer provides more high frequency boost than necessary to cancel the cable loss, the resulting signal is over equalized. An over equalized signal contains ringing. FIGS. 6(a) and 6(b) illustrate an over equalization scenario.
In FIG. 6(a), a signal originating from a transmitter 604 travels over a cable 608 and is subjected to cable loss. The cable loss causes attenuation of the high frequency contents of the signal. An equalizer inside a receiver 612 attempts to restore the signal to its original shape. However, as shown in FIG. 6(a), the equalizer provides excess high frequency boost, which causes a ringing waveform.
FIG. 6(b) shows the equalization frequency response, cable loss characteristics and overall transfer function. Due to excessive equalization, the transfer function indicates that the resulting signal includes excessive high frequency boost, thus resulting in a ringing waveform.
Both under equalized and over equalized signals are undesirable in communication systems. An over equalized signal causes error in a peak detector, resulting in bit errors. An under equalized signal causes inter-symbol-interference (ISI). The problems caused by under equalization and over equalization will be discussed further below.
FIGS. 7(a)-7(c) illustrate the problems due to over equalization. In FIG. 7(a), the original waveform consists of 0, 1, 0, −1. FIG. 7(b) illustrates the resulting waveform after over equalization. As seen in FIG. 7(b), due to over equalization, the waveform contains significant ringing. FIG. 7(c) illustrates the waveform after being processed by a slicer. The output of the slicer is an incorrectly restored signal consisting of 1 0, 1, −1, 0, 1, −1, 0, −1, 1. Thus, the output of the slicer contains significant error.
FIGS. 8(a)-8(c) illustrate the problem due to under equalization. In FIG. 8(a), the original waveform consists of 0, 1, 0, −1. FIG. 8(b) illustrates the resulting waveform after under equalization. FIG. 8(c) illustrates the waveform after being processed by a slicer. The output of the slicer consists of 1 0, −1, 0. Thus, the output of the slicer contains significant error.
In general, two types of equalization schemes can be used to restore a signal: fixed equalization; and adaptive equalization.
In a fixed equalization, the equalizer compensates the signal with a predetermined constant or fixed boost of the signal's high frequency content. Although the fixed equalization scheme is simple and reliable, because the equalization is predetermined and fixed, this scheme is only suitable in one optimized situation. For example, if the equalizer is optimized for a 500 feet cable, then for all other situations, the equalizer is sub-optimal, i.e., over equalized or under equalized. Thus, the equalizer optimized for a 500 feet cable is over equalized for a cable length less than 500 feet, and is under equalized for a cable length greater than 500 feet.
In an adaptive equalization, the amount of equalization is determined dynamically rather than being predetermined and fixed. The adaptive equalization scheme utilizes a nonlinear algorithm wherein the equalizer continuously monitors a channel characteristics or behavior and tries to cancel the cable loss with an optimal equalization. If the channel characteristics change, the equalizer adapts to the new channel characteristic by adjusting the amount of equalization, and restores the signal correctly.
However, adaptive equalization scheme suffers from a disadvantage common to all non-linear algorithms. The adaptive equalizer can be trapped in a failure state (i.e., incorrect state) and not able to recover from the failure state. If the adaptive equalizer is trapped in a failure state, the equalization will be under equalized or over equalized, thus resulting in errors.
For example, if the equalizer is over equalized thus causing the CDR 220 to lose lock, the clock signal supplied by the CDR 220 will have incorrect timing. The incorrect timing causes incorrect sampling of the signal, resulting in false information regarding the signal. If the sampling behavior incorrectly indicates that the waveform is under equalized when in reality it is over equalized, the equalizer's coefficient is increased until it reaches a maximum, causing the adaptive equalization to be trapped in an incorrect state.
Accordingly, there is a need for an adaptive equalization scheme that provides optimum level of equalization. There is a need for an adaptive equalization scheme that detects when the equalizer is trapped in a failure state. There is also a need for an adaptive equalization scheme that allows the equalizer to recover from the failure state.